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THE MACH SYSTEM

"Operating Systems Concepts, Sixth Edition" by Abraham Silberschatz, Peter Baer Galvin, and Greg Gagne

Presentation by Betsy Kavali

Mach is a Microkernel OSPicture from Wikipedia

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History of Mach

Derived its communication system and philosophy from Accent.

BSD Unix support➢ Originally constructed inside 4.2BSD kernel.➢ Replaced one piece at a time.

Started with an effort to support multiprocessors.

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Goals of Mach

➢ Support diverse architectures

- UMA, NUMA, NORMA➢ Simplified kernel structure➢ Compatibility with UNIX, Ease of use.➢ Integrate memory management and IPC➢ Distributed Operation and Varying network speed➢ Heterogeneous System support.➢ Object-oriented design

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System Components

• Task• Thread• Port• Port set• Message• Memory object

task

text region

threadsport

port set

message

data region

memory object

secondarystorage

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System Components

Task: Execution environment Contains one or more threads Provides a protection domain, a protected access to system resources via portsThread: unit of computation (execution) must run in the context of a task all threads in a task share ports, memory, etc. process = task + thread

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System ComponentsPort: Kernel protected communication channel.Mechanism to reference an object.Port set:Group of ports sharing a common message queueMessage:Basic method of communication between threads in different tasks.Memory objects: storage unit,Map all or part of object into address spaceThey are accessed by tasks using ports

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Process Management

Tasks ➢ Parent task creates children tasks➢ New Tasks contain one thread initially.➢ Suspending a task, suspends threads in the task.Threads➢ Suspending/resuming a thread does not suspend/resume the task.➢ Threads share the address space of the task , hence the need for synchronization

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Process Management - Threads

User Level Threads➢ Mach provides a basic kernel interface for managing

threads➢ C threads package is built on top of Mach's

primitives. Influenced POSIX P threads standard.

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Process Management - Threads

Thread control routines :

Create : give function to execute and its parameters

Destroy : Destroys the thread and returns a value to the creating thread.

Wait : for a specific thread to terminate then continue the calling thread

Yield : Thread yields use of a processor.

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Process Management - Threads

Mutual Exclusion using spin locks. Mutual exclusion Routines are:

Mutex_lock, mutex_unlock, mutex_alloc, mutex_free. Synchronization through condition variables(wait,

signal), the associated routines are: Condition_alloc, condition_free

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CPU Scheduler➢ Only threads are scheduled, tasks are ignored.➢ Each thread will have a priority number(0 -127)➢ Dynamic thread priority - The lowest priority thread is the

one with the most recent large CPU usage.➢ Global run queues + per processor local run queues➢ Processors consult run queues to select next thread:

the local queue first, then the global queue➢ Thread time quantum varies inversely with total

number of threads

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Exception Handling➢ The exception handler is just another thread in the task.➢ RPC messages: synchronize & communicate between victim

and handler.

Two different granularities of exception handling.

Error Handlers: Perform recovery actions in response to an exception and resume execution of the thread.

Debuggers: Examine the state of an entire application to investigate why an exception occurred and/or why the program is misbehaving.

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Exception handling proceeds as follows.

1. Victim: raise – RPC message sent to the handler.

2. Victim: wait -- synchronize with completion of exception

handling.

3. Handler: catch -- receive notification, identifies the

exception and the victim

4. Handler: take actions

• clear -- clear exception causing victim to return from wait.

• terminate -- cause termination of victim thread.

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InterProcess Communication - IPC

Location Independent IPC

The two components of IPC are

1. Ports

2. Messages

Ports :

Protected bounded queue within the kernel

Capability: send or receive ``right‘’

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InterProcess Communication - IPC

System calls for port functionality.

Allocate: new port in a task(task decides the rights of the port)

Deallocate: revoke tasks access rights to a port.

Get current port status.

Create a back up port

-port sets : Useful when one thread has to service requests coming on multiple ports.

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InterProcess Communication - IPC

Messages :

Header + one or more typed data objects

Header : contains destination port name, reply port

name, message length

In-line message data : typed data, port rights

Out-of-line data: pointers to data

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InterProcess Communication - IPCNetMsgServer Used when receiver port is not on the kernel’s

computer User-level daemon that forwards messages

between hosts Provides Name Service Primitive -Allows tasks

networkwide to register ports for lookup It is protocol independent.

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NetMsgServer

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Memory Management.

Memory Object: Mach's basic abstraction of physical memory, an

object. Secondary storage or data that are mapped into

virtual-memory (Files, pipes) Served by user-Level memory managers.

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Memory management

User-Level memory manager -

Memory can be paged by user-written memory managers

Mach has no knowledge of memory object contents.

Default memory manager - Used in circumstances when there is no local manager.

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Blend of Memory and IPC

This is an unique feature of Mach, and key to the system's efficiency.

Memory management using IPC.

A memory object is represented as a port.

To request operations on this object, IPC messages are sent to this port.

Because IPC is used, memory object may reside on remote systems, Kernel caches the contents.

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Blend of Memory and IPC

IPC using memory management techniques:

Here messages are passed by moving pointers to shared-memory objects.Virtual-memory remapping to transfer large contents using virtual copy / copy-on-write techniques

Virtual copy/ copy on write http://www.sci.csuhayward.edu/~billard/cs4560/node23.html

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Programmer Interface

System-call level:Implemented via emulation libraries and serversC Threads package:C language interface to Mach threads primitivesNot suitable for NORMA systems

Interface/Stub generator (MIG):Input = Interface definition (declarations of variables, types & procedures)Output = RPC interface code

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Conclusion

➢ Micro kernel

➢ Few simple abstractions

➢Higher level OS functionality built in user level servers➢ Focus on communication facilities

➢ Mach pioneered many concepts.

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Thank You